The invention relates to a multiple-output digital to analog converter as defined in the precharacterizing part of claim 1. The invention further relates to a method of converting digital data into a plurality of analog output voltages as defined in the precharacterizing part of claim 8. The invention also relates to a matrix display with such a converter as defined in the precharacterizing part of claim 9.
Such a multiple-output digital to analog converter is known from prior art U.S. Pat. No. 4,766,430 which discloses a drive circuit for a liquid crystal display with a matrix of picture elements. Row select lines select the picture elements row by row and a drive circuit drives the picture elements in a selected row in parallel by supplying drive voltages via column data lines. The drive circuit receives an input information signal representative of the data to be displayed to supply digital brightness counts, individually indicative of brightness levels for each picture element, to counters, whereby there is one counter for each column of the display. Each counter is first loaded with the brightness count and subsequently starts counting down. The counter turns off a transfer gate the instant its count becomes zero. As long as the transfer gate is on, the associated picture element receives a ramping voltage generated by an analog waveform generator. The instant the count reaches zero, the transfer gate disconnects the picture element from the waveform generator and the voltage across the picture element is frozen. In this way, the brightness of the picture element is controlled by the brightness count loaded into the counter, and is determined by the value of the ramping voltage the instant the count reaches zero.
It is a drawback of the drive circuit or the multiple-output digital to analog converter which converts digital brightness counts into analog voltages across the picture elements that the ramping voltage is disturbed the instant a transfer gate disconnects a picture element. This disturbance is especially large if many picture elements have to display a same brightness. The disturbance of the ramping voltage causes a visible brightness error for picture elements which are disconnected only a short time later.